Conventional telephony networks offer a service which reserves 64 kbits−1 of the network's capacity for the duration of a user's call. This service is suitable for voice telephony, where a signal representing the voice of the user must be continuously sent across the network to the other party to the conversation. In the trunk links of the telephony network, a number of telephone calls are carried on the link, each having its own timeslot in a sequence of timeslots. This is known in the art as time division multiplexing.
In contrast to telephones, computers send data in bursts (e.g. a Web server might send a page at 1 Mbits−1 for a few milliseconds once every minute on average). Reserving 1 Mbits−1 permanently for communication from a computer would be inefficient. For this reason, dedicated computer networks offer a different type of service. They do not reserve capacity for a user, but instead offer a service where the resources of the network (or at least a proportion of those resources) are shared indiscriminately between network users. This is known in the art as statistical multiplexing.
The last two decades have seen the introduction of so-called ‘integrated’ networks which are designed to carry both computer communications and telephony. The capacity of those networks therefore needs to be allocated between users who require a constant bit-rate for the duration of a communication (e.g. telephone users) and those who can tolerate some variation in the bit-rate supplied to them by the network (e.g. those transferring web-pages, software or e-mails).
In order to accommodate the different requirements of different types of communication, one type of integrated network, an Asynchronous Transfer Mode (ATM) network, allows a user to choose between a constant bit-rate service type (in which an amount of bandwidth they request is allocated to them for the duration of a session), a variable bit-rate service type (in which a proportion of the available capacity is shared between a number of users controlled to substantially prevent congestion), and an unspecified bit-rate service type (in which the remaining capacity is simply shared by all users). In the first two types of service, a user can additionally specify the amount of bandwidth he or she wishes to have available. Note that ‘service’ is to be distinguished from ‘service type’—a specification of a required service will include both a service type and an indication of the amount of network resources required by the user.
Similarly, another type of integrated network, an Internet Protocol (IP) network, can also offer a constant bit-rate service type (using the Resource Reservation Protocol (RSVP)), and a best efforts service type which approximates to the unspecified bit-rate service offered by ATM networks. Another service type gives packets sent by one class of users priority over packets sent by another class of user. Using RSVP, a user can additionally specify the amount of bandwidth he or she wishes to have available.